The use of amorphous polymeric fibers in nonwoven fibrous webs often requires undesirable compromises in processing steps or product features. Known amorphous polymeric fibers are formed under conditions that result in uniform thermal properties (e.g., glass transition temperature) throughout the fibers. The uniform thermal properties of the fibers results in essentially simultaneous softening, thereby causing substantially the entire fiber to coalesce into a mass of polymer that loses its fibrous shape within a very small temperature range. Because the amorphous polymeric fibers lose their fibrous shape, during heat bonding, nonwoven fibrous webs that include known amorphous polymeric fibers must typically also include one or more components to assist with bonding or to provide a fibrous nature to the web.
For example, some nonwoven fibrous webs that include amorphous polymeric fibers as a predominant fiber in their construction may rely on the use of binders or other materials to bond the amorphous polymeric fibers within the web, thereby eliminating the need to heat the web to a temperature sufficient to soften and coalesce the amorphous polymeric fibers contained within the web. Disadvantages of this approach may include, however, the processing issues associated with applying and curing or drying the binder material. Another potential disadvantage is that the web includes materials other than the amorphous polymeric fibers, which may complicate recycling of the nonwoven webs due to the need to separate the different materials used in the finished web. Still another disadvantage is that the binder may leave the web more paperlike, stiff, brittle, etc. Furthermore, the binder may reduce the breathability of the web by at least partially occupying the interstices between the fibers of the web.
Some nonwoven fibrous webs include amorphous polymeric fibers mixed with other non-amorphous polymeric fibers, with the amorphous polymeric fibers being provided as a bonding agent. For example, the web may include non-amorphous polymeric fibers made of semicrystalline polymers, cotton, cellulose, etc., in addition to amorphous polymeric fibers. In these nonwoven fibrous webs, the amorphous polymeric fibers may be provided as a bonding agent, with the intent that the amorphous polymeric fibers, when heated, coalesce into masses of polymer that bind the other fibers together within the web. Nonwoven fibrous webs with such a construction may be point-bonded or wide area calendered. Wherever sufficient heat and pressure is applied to result in softening of the amorphous polymeric fibers within the web, the amorphous polymeric fibers will typically be substantially nonexistent because the amorphous polymeric fibers will have typically all coalesced to form the bonds between the other fibers within the web. For example, within the area occupied by a point bond, substantially all of the amorphous polymeric fibers will have coalesced to form the bond.
As with the use of separate binder materials, the use of amorphous polymeric fibers in combination with other fibers may increase the cost of the web, make the manufacturing operation more complex, and introduce extraneous ingredients into the webs. Further, the heat and pressure used to form the bonds can change the properties of the web, making it, e.g., more paperlike, stiff, or brittle.